.\" Copyright (C) The libssh2 project and its contributors.
.\" SPDX-License-Identifier: BSD-3-Clause
.TH libssh2_session_callback_set 3 "1 Jun 2007" "libssh2 0.15" "libssh2"
.SH NAME
libssh2_session_callback_set - set a callback function
.SH SYNOPSIS
.nf
#include <libssh2.h>

void *
libssh2_session_callback_set(LIBSSH2_SESSION *session,
                             int cbtype, void *callback);
.fi
.SH DESCRIPTION
Sets a custom callback handler for a previously initialized session
object. Callbacks are triggered by the receipt of special packets at the
Transport layer. To disable a callback, set it to NULL.

\fIsession\fP - Session instance as returned by
.BR libssh2_session_init_ex(3)

\fIcbtype\fP - Callback type. One of the types listed in Callback Types.

\fIcallback\fP - Pointer to custom callback function. The prototype for
this function must match the associated callback declaration macro.
.SH CALLBACK TYPES
.IP LIBSSH2_CALLBACK_IGNORE
Called when a SSH_MSG_IGNORE message is received
.IP LIBSSH2_CALLBACK_DEBUG
Called when a SSH_MSG_DEBUG message is received
.IP LIBSSH2_CALLBACK_DISCONNECT
Called when a SSH_MSG_DISCONNECT message is received
.IP LIBSSH2_CALLBACK_MACERROR
Called when a mismatched MAC has been detected in the transport layer. If the
function returns 0, the packet will be accepted nonetheless.
.IP LIBSSH2_CALLBACK_X11
Called when an X11 connection has been accepted
.IP LIBSSH2_CALLBACK_SEND
Called when libssh2 wants to send data on the connection. Can be set to a
custom function to handle I/O your own way.

The prototype of the callback:

.nf
ssize_t sendcb(libssh2_socket_t sockfd, const void *buffer,
               size_t length, int flags, void **abstract);
.fi

\fBsockfd\fP is the socket to write to, \fBbuffer\fP points to the data to
send, \fBlength\fP is the size of the data, \fBflags\fP is the flags that
would have been used to a \fIsend()\fP call and \fBabstract\fP is a pointer
to the abstract pointer set in the \fIlibssh2_session_init_ex(3)\fP call.

The callback returns the number of bytes sent, or \-1 for error. The special
return code \fB-EAGAIN\fP can be returned to signal that the send was aborted
to prevent getting blocked and it needs to be called again.
.IP LIBSSH2_CALLBACK_RECV
Called when libssh2 wants to read data from the connection. Can be set to a
custom function to handle I/O your own way.

The prototype of the callback:

.nf
ssize_t recvcb(libssh2_socket_t sockfd, void *buffer,
               size_t length, int flags, void **abstract);
.fi

\fBsockfd\fP is the socket to read from, \fBbuffer\fP where to store received
data into, \fBlength\fP is the size of the buffer, \fBflags\fP is the flags
that would have been used to a \fIrecv()\fP call and \fBabstract\fP is a pointer
to the abstract pointer set in the \fIlibssh2_session_init_ex(3)\fP call.

The callback returns the number of bytes read, or \-1 for error. The special
return code \fB-EAGAIN\fP can be returned to signal that the read was aborted
to prevent getting blocked and it needs to be called again.
.IP LIBSSH2_CALLBACK_AUTHAGENT
Called during authentication process to allow the client to connect to the
ssh-agent and perform any setup, such as configuring the agent or adding keys.

The prototype of the callback:

.nf
void authagent(LIBSSH2_SESSION* session, LIBSSH2_CHANNEL *channel,
               void **abstract);
.fi
.IP LIBSSH2_CALLBACK_AUTHAGENT_IDENTITIES
Not called by libssh2. The client is responsible for calling this method when
a SSH2_AGENTC_REQUEST_IDENTITIES message has been received.

The prototype of the callback:

.nf
void identities(LIBSSH2_SESSION* session, void *buffer,
                const char *agent_path,
                void **abstract)
.fi

\fBbuffer\fP must be filled in by the callback. Different clients may implement
this differently. For example, one client may pass in an unsigned char ** for
this parameter, while another may pass in a pointer to a struct.

Regardless of the type of buffer used, the client will need to send back a list
of identities in the following format.

uint32 buffer length
uint32 number of entries
entries

Where each entry in the entries list is of the format:

string data
cstring comment

\fBagent_path\fP The path to a running ssh-agent on the client machine, from
which identities can be listed.
.IP LIBSSH2_CALLBACK_AUTHAGENT_SIGN
Not called by libssh2. The client is responsible for calling this method when
a SSH2_AGENTC_SIGN_REQUEST message has been received.

The prototype of the callback:

.nf
void sign(LIBSSH2_SESSION* session,
          unsigned char *blob, unsigned int blen,
          const unsigned char *data, unsigned int dlen,
          unsigned char **sig, unsigned int *sig_len,
          const char *agent_path,
          void **abstract);
.fi

When interfacing with an ssh-agent installed on the client system, this method
can call libssh2_agent_sign(3) to perform signing.

.SH RETURN VALUE
Pointer to previous callback handler. Returns NULL if no prior callback
handler was set or the callback type was unknown.
.SH SEE ALSO
.BR libssh2_session_init_ex(3)
.BR libssh2_agent_sign(3)
